Effective methods for the recovery and/or separation of particular divalent metal ions, such as (a) Ca2+ from source solutions containing Mg2+ and/or other ions and (b) Cd2+ from source solutions containing Zn2+ and/or other ions, are of great importance in modern technology. It is particularly difficult to remove these particular metal ions in the presence of moderate to strong acids and soluble complexing or chelating agents, such as the halide ions, which have a high affinity for the desired metal ions. It is also difficult to remove the aforementioned divalent metal ions when they are present at low concentrations in solutions containing other metal ions at much greater concentrations. Hence, there is a real need for a process to selectively concentrate certain divalent metal ions when present at low concentrations and particularly when in the presence of acid solutions and other complexing agents.
Some methods for the recovery of divalent metal cations from solution are known in the art. However, the removal and/or separation of specific divalent metal cations is often complicated by a myriad of conditions including the presence of other divalent cations, the presence of other cations in very high concentrations and the presence of other chelating agents. It is also known that ligands present as solutes in a solvent have the ability to selectively form strong bonds with particular ions or groups of ions present as solutes in the same solvent according to size, donor atom-related properties and other known selectivity characteristics.
Ethyleneglycol-bis-(xcex2-aminoethyl ether)-N,N,N,Nxe2x80x2-tetraacetic acid and (ethylenediamine)tetraacetic acid, commonly referred to as EGTA and EDTA respectively, are both ligands that may be used as solutes to complex divalent ions. However, EGTA has a greater selective preference for larger divalent metal ions than EDTA. See Critical Stability Constants, Volume 1: Amino Acids, A. E. Martell and R. M. Smith, Plenum Press, N.Y. and London, 1974. Therefore, EGTA is an important ligand for use in complexing larger divalent metal ions.
This being the case, researchers have not previously been able to incorporate EGTA into phase separation systems. This is significant because EGTA present in solution as a solute simply acts to complex selected ions, but does not provide a means for their separation. Specifically, never before has EGTA been successfully covalently bonded to inorganic and/or organic solid supports. As such, EGTA bonded to solid supports have not been used in phase separation processes for removing, separating and concentrating larger ionic radii or desired divalent ions from solutions, particularly where such desired ions are present in solutions with smaller and/or similar ions present at a much higher concentration.
As such, it would be useful to provide a composition and method for removing, separating, and/or concentrating certain desired divalent metal ions in solution from other ions, such as (a) Ca2+ from Mg2+ and/or (b) Cd2+ from Zn2+, even when the desired ion is present at a low concentration.
The present invention is drawn to novel porous and/or non-porous particulate organic and/or inorganic solid supports containing EGTA ligands covalently bonded to the solid support through a spacer. When the particulate solid support is an organic resin or polymer, the EGTA ligand may be bonded directly to an activated polar group on the polymer. The invention is also drawn to methods for removing, separating, and/or concentrating certain desired divalent metal ions including (a) Ca2+ from source solutions containing Mg2+ and/or other ions and (b) Cd2+ from source solutions containing Zn2+ and/or other ions. In fact, the removal of these ions (Cd2+ and/or Ca2)may occur when they are present at from very low to very high concentrations, i.e., from ppb to g/l levels of Cd2+ and/or Ca2+.
The concentration of the desired ions is accomplished by forming a complex of the desired ions with an EGTA ligand bound solid support material by flowing a source solution containing the desired ions through a column or other separation device packed with EGTA ligand bound solid support material. This process enables the desired ions to complex with the EGTA ligand attached to the solid.support material. The metal ion and the EGTA ligand are then decoupled by flowing a receiving liquid through the column or other separation device (in much smaller volume than the volume of source solution passed through the column) to remove and concentrate the desired ions in the receiving liquid solution. The receiving liquid or recovery solution forms a stronger complex with the desired ions than does the EGTA, or alternatively, temporarily forms a stronger interaction with the EGTA ligand than do the desired metal ions, and thus, the desired metal ions are quantitatively stripped from the ligand in a concentrated form in the receiving solution. The recovery of desired ions from the receiving liquid may be accomplished by various methods commonly known in the art.
The present invention provides a composition and method for the removal and/or separation of particular divalent metal ions including Ca2+ and Cd2+ present in low concentrations from a solution utilizing an EGTA ligand attached to a solid support. The present invention is particularly adaptable to the removal of (a) Cd2+ from source solutions containing Zn2+ and/or other ions and (b) Ca2+ from source solutions containing Mg2+ and/or other ions. Further, the invention may be carried out in moderately acidic solutions, e.g., solutions with a pH greater than about 3.0. Furthermore, the desired selectivity and interaction strength is unexpectedly high despite the fact that one of the four carboxylic acid groups of the ligand is used to attach the ligand to the solid support.
The present invention requires that the EGTA ligand be covalently bonded to an organic polymeric or inorganic particulate solid support that can be porous or nonporous. If an inorganic solid support is used, a hydrophilic spacer is linked to a silicon, carbon, nitrogen, oxygen or sulfur atom and is further covalently bonded to the solid support. If an organic polymer or resin solid support is used, the ligand can contain a functional group reactive with a constituent of the polymer (such as an activated polar group). In either case, the structure may be represented by Formula 1, as follows:
xe2x80x83SSxe2x80x94Axe2x80x94Xxe2x80x94Lxe2x80x83xe2x80x83Formula 1
where SS is a porous or non-porous particulate solid support, A is a covalent linkage mechanism, X is a hydrophilic spacer grouping and L is an EGTA ligand. If an organic resin or polymer is used for the solid support, the covalent linkage A and spacer X will then be integrated, and may actually be a single linkage, formed by the covalent bond formed by the reaction between the activated polar group from the polymer and the functional group from the ligand.
The SSxe2x80x94Axe2x80x94Xxe2x80x94 portion of Formula 1 is well known for use with ion binding ligands. Preferably, the solid support xe2x80x9cSSxe2x80x9d is an inorganic and/or organic particulate support material selected from the group consisting of sand, silicas, silicates, silica gel, glass, glass beads, glass fibers, alumina, zirconia, titania, nickel oxide, polyacrylate, polystyrene, polyphenol, combinations thereof, as well as any others described in U.S. Pat. Nos. 4,943,375, 4,952,321, 4,959,153, 4,960,882, 5,039,419, 5,071,819, 5,078,978, 5,084,430, 5,173,470, 5,179,213, 5,182,251, 5,190,661, 5,244,856, 5,273,660 and 5,393,892, which are herein incorporated by reference. However, other organic resins or any other hydrophilic organic and/or inorganic support materials meeting the above criteria can also be used.
The use of one or more ion binding ligands attached to an SSxe2x80x94Axe2x80x94Xxe2x80x94 solid support by a covalent linkage spacer group is illustrated in U.S. Pat. Nos. 4,943,375, 4,952,321, 4,959,153, 4,960,882, 5,039,419, 5,071,819, 5,078,978, 5,084,430, 5,173,470, 5,179,213, 5,182,251, 5,190,661, 5,244,856, 5,273,660 and 5,393,892. These patents, which disclose various spacers that can be used in forming an organic ligand attached to a solid support, are incorporated herein by reference. However, a preferred method of attaching the EGTA ligand to the solid support is through one of the carboxyl groups of the EGTA to form an amide.
When the solid support SS is an inorganic material such as silica, silica gel, silicates, zirconia, titania, alumina, nickel oxide and glass beads, the covalent linkage A is a silane such that Axe2x80x94X may be represented by Formula 2 below: 
In the above formula, the A portion is Si(Z,Zxe2x80x2)xe2x80x94O wherein Z and Zxe2x80x2 can independently represent members selected from the group consisting of Cl, Br, I, alkyl, alkoxy, substituted alkyl or substituted alkoxy and Oxe2x80x94SS (when SS is an inorganic solid support). When Z and Zxe2x80x2 moieties are other than Oxe2x80x94SS they are functionally classified as leaving groups, i.e. groups attached to the silicon atom which, when reacted with an Qxe2x80x94SS material, may leave or be replaced by the Oxe2x80x94SS. Alkyl or alkoxy preferably mean 1-6 carbon member alkyl or alkoxy group which may be substituted or unsubstituted, straight or branched chain. Substituted groups can include Cl, Br, I, NO2 and the like. The X portion is a spacer grouping is preferably represented by Formula 3 below:
(CH2)mO(CH2)n(Q)p(CH2)q[NH(CH2)sNH2]txe2x80x83xe2x80x83Formula 3
wherein m, n, q and s are independently integers from about 1 to 20; Q is oxygen(O), sulfur(S) or nitrogen(N); p is 0 or 1; and t is from 0 to 5. It is to be noted that each s variable may be independently an integer from about 1 to 20 regardless of the number of t groups. For example, if t is 3, each of the three s variables represented may be independently an integer from about 1 to 20.
When the SS is a particulate polymeric organic solid support matrix such as polyacrylate, polystyrene, polyphenol and combinations thereof, the A and X elements of Formula 1 may be combined and therefore, may be represented by Formula 4 below:
xe2x80x94(CH2)xxe2x80x94(Y)yxe2x80x94(CH2)zxe2x80x83xe2x80x83Formula 4
where y is 0 or 1; x and z are independently from about 0 to 10;
and Y is member selected from the group consisting of O, S, Cxe2x95x90N, CO, CONH, CSNH, COO, CSO, NH, NR, SO, SO2, SO2NH, C6H4 and CH2C6H4, where R is lower alkyl with the proviso that at least one of x, y and z must be at least 1.
It is to be emphasized that the present invention does not reside in the discovery of the SSxe2x80x94Axe2x80x94Xxe2x80x94 portion of Formula 1. Rather, it is the discovery that the ion-binding capabilities of the EGTA ligand (L) when attached to an SSxe2x80x94Axe2x80x94X based solid substrate are highly functional for selective ion separation of one species from another.
EGTA ligands covalently bonded to solid supports as shown in Formula 1 are characterized by high selectivity for and removal and/or separation of desired divalent metal ions such as (a) Ca2+ from source solutions containing Mg2+ and/ other ions, and (b) Cd2+ from source solutions containing Zn2+ and/or other ions.
The present invention also provides methods for removing desired divalent metal ions from solutions that can contain other metal ions present in much higher concentrations by the use of EGTA ligand bonded solid support materials. The separation is accomplished, even in the presence of other complexing agents or matrix constituents in a separation device, such as a packed column, through which the solution is flowed causing a complex of the desired ions to be formed with ligand bound solid support material packed within the column. The process of selectively removing and concentrating the desired ions is characterized by the ability to quantitatively complex from a larger volume of solution the desired ions, even when they are present at very low concentrations.
The desired ions are recovered from the ligands bound to the solid supports by flowing through a small volume of a receiving phase which contains a solubilizing reagent. The solubilizing reagent need not be selective, but merely be capable of forming a stronger complex with the desired divalent metal ion than does the EGTA ligand. Alternatively, the solubilizing reagent may contain ions that temporarily form a stronger interaction with the EGTA ligand than does the desired metal ions. This causes the desired metal ion to be quantitatively stripped from the solid support material in concentrated form and transferred to the receiving solution. The recovery of the desired metal ion from such a receiving solution is readily accomplished by evaporation, precipitation or by other known methods.
There are several applications for which one may wish to remove/separate the aforementioned divalent metal ions from solution. An example of such an application is the separation of ppb levels of Cd2+ or other divalent metals from concentrated Cu, Ni, Zn, Ag, Pb and/or Hg contained in process streams and industrial effluents. This is true both because of the unsatisfactory technologies presently available or because a more economical method is desired. For example, Cd2+ is often present at low concentrations in solutions containing other divalent ions at much greater concentrations. Therefore, the removal of Cd2+ from such solutions would be important environmentally because of impurity concerns. Additionally, quantitation and/or removal of Cd2+ or Ca2+ ions from other non-desirable ions may also be employed. Specifically, this removal process may be implemented when the desired ions (Cd2+ and/or Ca2+) are present at from very low levels (ppb) to very high levels (g/l), even in the presence of concentrated Cu, Ni, Zn, Ag, Pb and/or Hg process streams.